Invasive fungal sinusitis is a potentially lethal disease that causes painful orbital apex syndrome with ophthalmoplegia and visual loss. The mechanism of ophthalmoplegia and visual loss is not clear, and neither is the role of radical surgery-orbital exenteration. The authors report a single case of an immunocompromised patient with invasive fungal sinusitis in whom the visual loss was due to posterior ischaemic optic neuropathy, presumed from fungal invasion of small meningeal-based arteries at the orbital apex. Following intensive antifungal drugs, orbital exenteration and immune reconstitution, the patient survived. However, there is uncertainty around the benefit of exenteration and whether this radical treatment helped the final outcome. There was little evidence of actual fungal invasion of the orbit. The authors therefore hypothesise that it is more likely the antifungal drugs saved this patient rather than the surgery. The authors suggest that the presence of acute posterior ischaemic optic neuropathy should be a contraindication to the need for orbital exenteration in this condition.